Is there a way in openldap for me to dynamically modify (preprocess)
an update? I want to calculate some attribute values on the server side.

If what you need to rewrite is DN values, you can try with the slapo-rwm
overlay (2.3) or back-ldap/back-meta (2.2); otherwise you need to write
your own plugin (overlay or SLAPI, that's a matter of taste).